The present relates to a linear accelerator.
In the use of radiotherapy to treat cancer and other ailments, a powerful beam of the appropriate radiation is directed at the area of the patient which is affected. This beam is apt to kill living cells in its path, hence its use against cancerous cells, and therefore it is highly desirable to ensure that the beam is correctly aimed. Failure to do so may result in the unnecessary destruction of healthy cells of the patient. Several methods are used to check this, and an important check is the use of a so-called xe2x80x9cportal imagexe2x80x9d. This is an image produced by placing a photographic plate or electronic imaging plate beneath the patient during a brief period of Irradiation. The beam is attenuated by the patient""s internal organs and structures, leaving an image in the plate. This can then be checked either before complete treatment or after a dose, to ensure that the aim was correct.
Portal images are however extremely difficult to interpret. The energy of the beam which is necessary to have a useful therapeutic effect is very much greater than that used for medical imaging. At these higher energies there is smaller ratio in the relative attenuation between bony and tissue structure, which results in portal images with poor contrast. Structures within the patient are difficult to discern.
Some existing radiotherapy devices include a second radiation source which is adapted to produce a lower energy beam for producing a portal image. This second source is usually placed either alongside the principal accelerator and parallel thereto, or is mounted at an angle such that the entire unit is rotated about the patient to bring the second source into line for the portal image, following which the unit is rotated back for treatment. Both arrangements present difficulties in ensuring adequate alignment between the principal accelerator and the second source.
It has not hitherto been possible simply to reduce the energy of the principal (therapeutic) accelerator, since this must operate in a relativistic mode in order to maintain beam quality. If the final beam energy is too low, then the beam will be non-relativistic at earlier parts of the accelerator, preventing satisfactory operation.
The present invention therefore provides an accelerator comprising a plurality of accelerating cells arranged to convey a beam, adjacent cells being linked by a coupling cell, the coupling cells being arranged to dictate the ratio of electric field in the respective adjacent accelerating cells, at least one coupling cell being switchable between a positive ratio and a negative ratio.
Such an accelerator is eminently suitable for therapeutic use as part of a radiotherapy apparatus as a phase change is in effect inserted into the E field by imposing a negative ratio meaning that the beam will meet a reversed electric field in subsequent cells and will in fact be decelerated. As a result, the beam can be developed and bunched in early cells while accelerating to and/or at relativistic energies, and then bled of energy in later cells to bring the beam energy down to (say) between 100 and 300 KeV. Despite this low output energy, the beam is relativistic over substantially the same length of the accelerator, as previously. Energies of this magnitude are comparable to diagnostic X-rays, where much higher contrast of bony structures exists. Hence the accelerator can be used to take kilovoltage portal images.
It is preferred that the switchable coupling cell comprises a cavity containing a conductive element rotatable about an axis transverse to the beam axis. This is more preferably as set out in our earlier application PCT/GB99/00187, to which specific reference is made and the contents of which are hereby incorporated by reference. Protection may be sought for features set out in this application in combination with features set out in that application.
The application likewise relates to the use of an accelerator in which a plurality of accelerating cells arranged to convey a beam, and adjacent cells are linked by a coupling cell, the coupling cells being arranged to dictate the ratio of electric field in the respective adjacent accelerating cells, wherein at least one coupling cell is switched between a positive ratio and a negative ratio.
Further, the application relates to an operating method for an accelerator in which a plurality of accelerating cells arranged to convey a beam, and adjacent cells are linked by a coupling cell, the coupling cells being arranged to dictate the ratio of electric field in the respective adjacent accelerating cells, wherein at least one coupling cell is switched between a positive ratio and a negative ratio.